1. Field of the Invention
The present invention relates to a head positioning control system which controls at least one head to be positioned on a target position of at least one recording medium such as a disk in at least one magnetic disk unit, optical disk unit or the like.
More specifically, the present invention relates to a head positioning control system which has a function of rapidly discriminating whether or not the head is positioned in the center of a target track of the disk where a track following operation can be performed for reading or writing data from or to the target track, when the target track is designated by a host unit, e.g., a host computer.
2. Description of the Related Art
There is recently a tendency to demand, in a computer system, a transfer of large amounts of data at high speed, and therefore, an auxiliary storage device such as a magnetic disk unit, optical disk unit, or the like, is also required to transfer large amounts of data at high speed to exchange data with the host computer. To meet this requirement, a magnetic disk unit or optical disk unit preferably includes a number of recording media, e.g., a number of disks on which read/write operations of data can be performed substantially simultaneously at high speed by switching the respectively corresponding heads at high speed, and the density of one of recording medium surfaces of a magnetic disk unit or optical disk unit must be increased (a track pitch of not greater than 10 .mu.m, for example).
In contrast, it is difficult to greatly reduce absolute quantities of eccentricity of each disk, deviation between heads, and so forth, which are caused by changes in ambient temperature, etc. In other words, the relative proportions of eccentricity, deviation, and so forth, appears to be increased. As a result, when one head by which a read/write operation has been completed is switched with another head, even though these two heads are located in the track positions of the respective disks in concentric relation with each other, forming of a cylinder (therefore, "track" is sometimes referred to as "cylinder"), the head after switching is likely to go off-track due to the above-mentioned eccentricity, deviation, and so forth.
Therefore, it is necessary to rapidly compensate for an off-track amount by a head positioning control system utilizing a servo control system so as to start performing a read/write operation by means of the head after switching. At this time, in order to confirm a function of the off-track compensation, it is also necessary to rapidly discriminate whether or not the head is positioned in the center of the track of the disk, i.e., whether or not the head is on-track.
Here, to enable the above-mentioned head positioning control system to be understood more clearly, a conventional control system by a servo surface servo system utilizing one surface of a disk as a servo surface will be described with reference to the related drawings of FIGS. 1 to 9, while another conventional control system by a data surface servo system, in which servo data are embedded in advance in all the surfaces of the disks, will be described with reference to the accompanying drawings of FIGS. 10 and 11. In each case, a head positioning control system is applied to a magnetic disk unit, which has a function of discriminating whether or not the head is on-track while the off-track compensation is being carried out.
FIGS. 1 to 9 are diagrams illustrating a conventional example according to a prior art, wherein FIG. 1 is a block diagram of an overall magnetic disk unit, FIG. 2 is a block diagram illustrating a disk mechanism, FIG. 3 is a block diagram of a head positioning servo control unit, FIG. 4 is a block diagram of an on-track discrimination circuit, FIG. 5 is a block diagram of an off-track compensation circuit, FIGS. 6(A) and 6(B) diagrams for explaining an off-track amount, FIG. 7 is a diagram for explaining a part of a process according to a prior art (part 1), FIG. 8 is a diagram for explaining the remaining part of a process according to a prior art (part 2), and FIG. 9 is a flowchart of the on-track discrimination process when the off-track condition is being compensated.
In FIGS. 1 to 9, reference numeral 100 denotes a magnetic disk unit, 200 denotes a host controller, 3 denotes a read/write control unit, 4 denotes a disk mechanism, 5 denotes a head positioning servo control unit, 6 denotes a spindle motor control unit, 7 denotes an off-track compensation unit, 9 denotes a VCM (voice coil motor), 10 denotes arms, 11 denotes a spindle, 12 denotes a servo head, 13 denotes a data head, 14 denotes data surfaces, 15 denotes a servo surface, 16 denotes a magnetic disk, 17 denotes an on-track discrimination circuit, 18 denotes a power amplifier, 19 denotes a speed control/position control switching circuit, 20 to 22 denote comparators, 23 denotes a position signal generating circuit, 24 denotes a velocity signal generating circuit, and reference numeral 25 denotes a servo block control unit.
Moreover, reference numeral 26 denotes a comparator circuit, 27 denotes a counter, 28 denotes a change detecting unit, 29 denotes an AND gate, 30 denotes a comparator circuit, 31 denotes a pulse generating circuit, 33 denotes an analog/digital converter (A/D converter), 34 denotes a work register, 35 denotes an arithmetic unit, 36 denotes an off-track compensation value register, and reference numeral 37 denotes an on-track slice register.
A magnetic disk unit employing a servo surface servo system has heretofore been known, as shown in FIGS. 1 to 5.
In such a magnetic disk unit, the position is not detected by the data head itself but is detected by the servo head only, which is mechanically coupled to the data head.
Therefore, the positions of these heads are deviated relative to each other due to a change in the ambient temperature during operation of the disk unit. Moreover, occurrence of an off-track situation essentially can not be avoided in such a disk unit.
To cope with this disadvantage, there has been proposed an off-track compensation technology which measures and compensates for an amount of the aforementioned deviation.
In order to compensate for the off-track amount, the amount of position deviation between the servo head and the data head is measured, and the data head is positioned so that an offset is generated in a direction cancelling the amount of deviation.
In a disk unit which has a plurality of data heads on an arm, furthermore, the deviation amount and the compensation amount vary depending upon the respective data heads.
That is, the heads must be finely moved, and the off-track compensation must be performed even during head switching operations. The off-track compensation must be frequently carried out because of variance in the off-track amounts among the heads, though high speed access seems to be expected for these switching operations since a seek command is involved in each switching operation.
Hereinafter, a conventional magnetic disk unit employing the above-mentioned servo surface servo system will be described in detail with reference to the above-mentioned drawings.
Referring to FIG. 1, the magnetic disk unit 100 is constituted by the host controller 200, read/write control unit 3, disk mechanism 4, head positioning servo control unit 5, spindle motor control unit 6, off-track compensation unit 7 and the like.
The host controller 200 performs a variety of control operations in the magnetic disk unit 1, exchanges various commands and data from the host unit, and controls the data.
The read/write control unit 3 reads and writes data from and to the disk mechanism 4 in response to an instruction from the host controller 200 and inputs and outputs read/write signals.
Upon receipt of a seek command from the host controller 200, the head positioning servo control unit 5 controls a head positioning operation in the disk mechanism 4.
The off-track compensation unit 7 receives off-track data from the read/write control unit 3 as well as head number and the like, from the host controller 200, and forms data for compensating for the off-track condition.
The spindle motor control unit 6 controls the revolution of a spindle motor in the disk mechanism 4.
The disk mechanism 4 is constituted, for example, as shown in FIG. 2.
In FIG. 2, the disk mechanism 4 is provided with a plurality of magnetic disks 16 which are coupled to a spindle 11 and are rotated by a spindle motor (not shown).
A servo surface 15 is formed on any one of the surfaces of these plurality of magnetic disks 16, and the other surfaces of the magnetic disks 16 are all data surfaces 14. A plurality of magnetic heads are provided on the arms (head arms) 10 that is driven by the VCM (voice coil motor) 9. An actuator is constituted by the arms 10 and the VCM 9.
Among these magnetic heads, the magnetic head on the side of the servo surface 15 is a servo head 12, and all the magnetic heads on the sides of the data surfaces 14 are data heads 13.
The servo head 12 reads servo data on the servo surface 15. The servo data that are read are sent to the head positioning servo control unit 5, and used for controlling the head positioning operation.
The data heads 13 read and write data from and to the data surface 14, and are controlled by the read/write control unit 3.
The VCM 9 is actuated by the head positioning servo control unit 5, and moves the data heads 13 and the servo head 12 in the radial direction of the magnetic disk 16 via the actuator 10.
The head positioning servo control unit 5 is constituted, for example, as shown in FIG. 3.
In FIG. 3, the servo block control unit 25 receives a seek command from the host controller 200, and controls the whole head positioning servo control unit.
The position signal generating circuit 23 demodulates the servo signal read by the servo head 12 and forms a position signal S.sub.PO.
The velocity signal generating circuit 24 receives the servo signal S.sub.SV and forms a velocity signal S.sub.V (actual velocity).
The comparator 21 finds a difference between an off-track compensation data (data from the off-track compensation unit 7) and a target position (instruction value) from the servo block control unit 25, and the comparator 20 finds a difference, i.e., a position error between the above difference of the comparator 21 and the position signal (actual position). The comparator 22 finds a difference, i.e., a velocity error between a target velocity (instruction value) from the servo block control unit 25 and the velocity signal (actual velocity) formed by the velocity signal generating circuit 24.
Upon receipt of a switching signal S.sub.SW from the servo block control unit 25, the velocity control/position control switching unit 19 switches the output (position error signal) of the comparator 20 and the output (velocity error signal) of the comparator 22, and sends the operation amount to the power amplifier 18.,
The power amplifier 18 drives the VCM 9 depending upon the output that corresponds to the operation amount in order to position the head.
The on-track discrimination circuit 17 inputs the on-track slice level from the off-track compensation unit 7, the head number from the host controller 2, and the output (position error signal) from the comparator 20, and discriminates whether or not the head is on-track.
The on-track discrimination circuit 17 is constituted, for example, as shown in FIG. 4 wherein the comparator circuit 30 compares the on-track slice level with the position error signal, and produces an output of a high level "1" when the position error is within the range of the on-track slice level, and produces an output of a low level "0" (the output is referred to as "on-track signal S.sub.ONT " that will be described later) when the position error lies outside the range of the on-track slice level.
The pulse generating circuit 31 outputs pulses of a predetermined period, and the AND gate 29 outputs the output pulses of the pulse generating circuit 31 as long as the output of the comparator circuit 30 is at a high level.
The change detecting unit 28 inputs the head number from the host controller 2 and detects a change such as switching of the heads.
The counter 27 counts the pulses output from the AND gates 29, and is reset when the output of the comparator circuit 30 becomes low level "0" or when the change detecting unit 28 has detected a change in the heads (e.g., switching of heads).
The comparator circuit 26 stores a reference value LO for comparison, and compares the counted value output from the counter 27 with the reference value LO (e.g., LO=5).
A track following signal S.sub.TF is output when the counted value has exceeded the reference value.
The off-track compensation unit 7 is constituted, for example, as shown in FIG. 5 wherein the analog/digital converter (A/D converter) 33 receives the off-track data from the read/write control unit 3 and converts it into a digital signal.
The arithmetic unit 35 performs a variety of arithmetic processes by using the work register 34 based on the digital off-track data, and outputs an off-track compensation value and an off-track slice level value.
The off-track compensation value register 36 inputs the head number from the host controller 200 and the off-track compensation value, and stores the off-track compensation value that corresponds to the head number.
The on-track slice register 37 inputs the head number from the host controller 200 and the on-track slice level, and stores the on-track slice level that corresponds to the head number.
Here, a track following signal can be exemplified as the signal for indicating a condition for permitting the read and write of data and for starting a track following operation. In a disk unit which does not use the above-mentioned off-track compensation technology, the condition where the track following operation is not carried out does not occur in principle even when the head switching operation is effected (even when a plurality of heads are switched), and therefore an on-track condition need not be discriminated.
When the off-track amount needs to be compensated due to the increase of track density, however, the individual heads are compensated for their off-track condition and are finely moved at the time of a head switching operation. That is, the condition where the track following operation is not carried out takes 5 place, and it becomes necessary to discriminate the aforementioned on-track condition.
Described below in detail is a conventional on-track discrimination process that is carried out while the off-track amount is being compensated.
In the magnetic disk unit of the servo surface servo system as described above, the data head is positioned based on the servo data read by the servo head.
For instance, the data is written by the data head 13 at a data head position that is determined with the servo track as a reference.
Therefore, the off-track amount is determined with the servo track center (center position of the servo track) as a reference.
FIG. 6(A) illustrates a case where the off-track amount is larger than the on-track slice level, and FIG. 6(B) illustrates a case where the off-track amount is smaller than the on-track slice level.
FIG. 6(A) illustrates a case where the servo head 12 has an on-track slice level of Ws, the head #1 (data head) exists at a position of an off-track amount "a" from the servo track center which is the center of the servo head 12, and the head #2 (data head) exists at a position of an off-track amount "b"
In this case, the off-track amounts "a" and "b" are greater than the on-track slice level Ws of the servo head 12.
FIG. 6(B) illustrates a case where the head #1 exists at a position of an off-track amount "c" from the servo track center and the head 2 exists at a position an off-track amount "d". In this case, the off-track amounts c and d are smaller than the on-track slice level Ws.
Described below are methods of discriminating the on-track condition while the off-track amount is being compensated.